I did it looking at CPU profiling support which is essentially the same.
The design doc also mentions functions that allow to emit user marks into the trace to denote scopes of something or just one-off events (a-la TraceEvent, TraceScopeBegin, TraceScopeEnd). I've already seen two inquiries about this on golang-nuts.
So StartTrace/EndTrace + TraceEvent/TraceScopeBegin/TraceScopeEnd can make a case for runtime/trace package.
People continue to ask about user-emitted events in traces. Some people also proposed some options for tracing, like disable stack collection.
We don't need to decide on all that for 1.5, I am sure we will have more trace-related functions in future, so it can make sense to move all trace-related functions to runtime/trace before the release.